This invention relates to a method of beneficiating graphite to higher purity levels by means of attrition.
Graphite, particularly flake graphite, is used in refractories, as a coating material in foundries, in brake linings, batteries, carbon brushes, as a precursor for expandable graphite, and as a lubricant.
In refractories, flake graphite imparts high thermal conductivity and improved slag resistance. During long term service at elevated temperatures the impurities in the graphite tend to degrade the high temperature stability of the flakes. Because of this there is an increasing tendency in refractory applications to use flake graphite of higher purity. For example, U.S. Pat. No. 4,912,068 describes the significant increase in hot strength made possible by using 99% pure flake graphite. Flake graphite sold to the battery market requires an impurity level of .ltoreq.300 ppm. Flake graphite sold to the expandable graphite market requires a purity of at least 96%. Large, high purity flakes command a higher price and are becoming increasingly desired. All of these tendencies are driving graphite producers to find new methods of producing high purity flake graphite.
Graphite of exceptional high purity can occur in veins and Sri Lankan vein graphite is an example of the same. Simple hand selection can be used to grade the lumps of such vein graphite into various purity levels.
In contrast, flake graphite occurs as flakes disseminated in a host rock and it is difficult to separate the flakes from the rock without destroying the size of the flakes and ensuring that the gangue is removed from the flakes to achieve a high level of purity. The rock containing the flake graphite must be ground to liberate the flake graphite and then the graphite is floated to separate the flakes from the rock and other impurities.
Such flotation is accomplished by introducing an aqueous slurry of the ground mass into a stirred cell wherein the free graphite is separated from the gangue by oil assisted buoyancy. Columns, or other beneficiation devices, such as tables, cyclones, or the like may also be used. The tails are then ground progressively finer to achieve liberation of the remaining flakes. This grinding further decreases flake size. To increase graphite purity, the concentrate must be further beneficiated. To achieve higher purity, especially in the fines (-100 mesh and finer fractions) the graphite can be chemically treated either with acids or alkalies or with a combination of the two. This process is expensive, time consuming, can modify flake properties, and can have a severe environmental impact. Another means to increase the purity of the concentrates is to heat them in a furnace at temperatures above 4000.degree. F. to vaporize the impurities.
The need exists to efficiently produce flake graphite at purity levels &gt;96% without removing the impurities by chemical or thermal processes and/or without significantly decreasing the size of the flakes. Much effort has been expended in recent years to accomplish these goals, such as evaluation of different milling techniques (pebble mills, rod mills, vertical stirred mills, etc.) but the desired combination of economically achieving high purity and large flakes has not been met.